docs: def last_modified(self, name: str = None) -> str:
  		"""
  		Return a compact ISO8601 timestamp (UTC timezone) indicating when an attribute was last modified
  
  		Note: if no attribute name is given (the default), the modification time of the most recently modified attribute will be returned
  		Note: if the attributes do not contain a timestamp, and the mode is 'r+', a new timestamp is created and returned.
  		Otherwise, the current time in UTC will be returned.
  		"""
  		a = ["/row_attrs/", "/col_attrs/"][self.axis]
  
  		if self.ds is not None:
  			if name is None:
  				if "last_modified" in self.ds._file[a].attrs:
  					return self.ds._file[a].attrs["last_modified"]
  				elif self.ds._file.mode == 'r+':
  					self.ds._file[a].attrs["last_modified"] = timestamp()
  					self.ds._file.flush()
  					return self.ds._file[a].attrs["last_modified"]
  			if name is not None:
  				if "last_modified" in self.ds._file[a + name].attrs:
  					return self.ds._file[a + name].attrs["last_modified"]
  				elif self.ds._file.mode == 'r+':
  					self.ds._file[a + name].attrs["last_modified"] = timestamp()
  					self.ds._file.flush()
  					return self.ds._file[a + name].attrs["last_modified"]
  		return timestamp()